The coating of substrates, for example glass surfaces, through sputtering/cathode sputtering in plasma processes, both in a reactive and conventional manner, is known, for example, from architecture glass coating. To this end, a plasma is generated by a current or voltage source, which removes material from a target, the material being deposited on the substrate, for example the glass pane. Before depositing, the atoms may bond to gas atoms or molecules in a reactive process, depending on the desired coating.
Medium frequency (MF) generators are frequently used, for example, in reactive processes, and are usually operated at a frequency of about 10 to 500 kHz. MF generators may be so-called freely oscillating MF generators or they may be MF generators with a fixed frequency. The freely oscillating MF generators usually include a closed-loop controlled or uncontrolled DC voltage, that is, the intermediate circuit voltage, which is generated from a single or multi-phase voltage of a mains power supply. The intermediate circuit voltage is converted into a medium frequency (MF) AC voltage (for example, about 10-500 kHz) using an inverter (for example, a bridge circuit). The MF output power signal is connected to an oscillating circuit, which is excited to oscillate. The inverter is thereby operated in such a manner that the oscillating circuit is operated close to the resonance frequency. The oscillating circuit may be a series oscillating circuit or a parallel oscillating circuit. A series oscillating circuit is excited by an output power signal with a voltage source characteristic, whereas the parallel circuit is excited with an output power signal with current source characteristic. To generate a current source characteristic from the intermediate circuit voltage, which usually has voltage source characteristics, chokes with sufficiently large inductance are connected in series with the inverter. The MF power is usually decoupled at the coil of the oscillating circuit and connected to two electrodes in a coating chamber of a coating system to permit plasma generation in the coating chamber. Decoupling may be effected using an output transformer that provides galvanic separation from the mains. The electrodes of an MF excitation system alternately operate as anode and cathode. The resonance frequency of the oscillating circuit depends on the impedance of the plasma. To ensure that the inverter is always operated close to the resonance frequency, the controller of the inverter monitors the MF output power signal. For example, such an arrangement is shown in DE 101 54 229 A1, which describes a means for closed-loop control of a plasma impedance.
In particular, in reactive processes, arcing may occur with MF generators, so-called micro-arcs, which are often automatically eliminate with the subsequent voltage reversal or at least after a few periods. Arcs of higher energy and longer duration may also occur. Arcs are typically detected by searching the output voltage for a voltage drop or by searching the output current for a current increase. Alternatively, an arc may be detected based on a difference between the currents to the individual electrodes. The user can set a limit value for detecting arcs in conventional systems. The effective current and voltage values are measured for the detection. In such a measurement, the magnitude of the voltage values and current values is integrated over the period to prevent zero crossings from being detected as voltage drops.
When MF generators are used in the semi-conductor production process, in particular, for producing flat panel displays FPD, the generators meet higher demands in that arcs must be detected within a few μs.